Income Taxes - Summary of Differences Between the Effective Tax Rate and the Statutory Federal Tax Rate (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Effective Income Tax Rate Reconciliation, Amount
Provision at statutory federal income tax rate									$ 148	$ 56	$ 49
Bank owned life insurance									(2)	(2)	(2)
State income tax (benefit), net of federal income tax effect (net of valuation allowance release)									18	(6)	(2)
Low income housing tax losses									(1)	(1)	(1)
Other									3	1	1
Total income tax expense	$ 51	$ 73	$ 32	$ 10	$ 11	$ 15	$ 14	$ 8	$ 166	$ 48	$ 45
Effective tax provision rate									23.50%	18.10%	19.40%